Cleaning native copper

Thiourea, as the name says, contains sulphur, and this is easily set free as sulphide in an aqueous solution. Sulphide, in turn, will attack copper over time (due to the extremely low solubility products of copper sulphides).

Also note that thiourea is an extremely strong mutagen that should be handled with as much care as possible ! Denaturing proteins with thiourea is easy as pie. Don't let the name fool you - the relation to urea is only a structural one, its properties are quite different.

First time user. I have some native copper that I have been trying to clean with muratic acid. I put several pieces in a gallon ice cream bucket, cover them with full strength acid overnight. Next I put them in a 5 gallon bucket of cold water and rinse them. Then I take them out one by one and scrub them with a toothbrush, rinse them off, towel dry, and put them in front of a heat lamp to dry with the intention of spraying them with hair spray or a lacquer. Before I can get them sprayed, the copper tarnishes, and the rock? material turns whitish/greenish/bluish. I tried soaking them in vinegar and coca-cola to brighten them up but to no avail. I put them in Tarn-x and it would make the copper a ....copper.....color, not shiny bright, but then by the time I rinse them off towel dry them to spray them they are tarnished again. I have then with some pieces resoaked them in muratic acid a second time but the second time they don't even bubble or steam. Some of the pieces are solid copper and some are copper imbedded in red rock, creme colored rock, green rock, black bubbly rock. I am getting frustrated at not being able to get any copper to stay shiny long enough to spray coat. When I tried soaking the specimens in cold water mixed with baking soda, the rocks came out looking kind of pinkish with a kind of slimy coating on them. Are there time limits for soaking the specimens in the muratic acid? Baking soda solution? (What ratio) How many rinses? Cold water? Warm water? Outside temperature make a difference? Time limit in Vinegar? Vinegar and salt? (Ratio) Coca-cola? Tarn-x? Copper Brite? Super iron out? Brasso? What is the proper sequence for doing these methods and the right length of time. I have been trying to do anywhere from 25-50 pieces at a time. Is that too many? I am 73 years old, retired and have a full schedule from spring to fall, so I try to get this done in the winter. By the first of March I will be heading to Arizona to get some more goodies so I have to work fast in the winter. I would appreciate any and all information and help I can get concerning this subject. Thank you. Bill

Can you show us a picture. Some of the guys who clean a lot of this stuff use an acid dichromate solution to make them really bright and shiny for the tourists. Collectors generally don't like real acid brightened copper specimens.

Now as far as cleaning your copper, I see two mistakes immediately. First, never leave the copper in muriatic acid overnight! This will “burn” the copper and destroy any crystals you have. I usually only leave any specimens in for 5 minutes at the very longest, and then only if there are no crystals on it (I do not use muriatic or any strong acid on crystalised specimens). Second, you are trying to do your cleaning in mass (25-50 pieces at a time); this is way too many at once. What is more than likely happening is you are putting so many pieces in that you are diluting the acid which changes the chemical makeup of the solution and will result in funky coloured pieces of copper. I will only do one or two at a time and if there is a lot of material to be removed, I will not re-use the acid, but rather start another round with fresh acid. And then there are just some pieces of copper, especially the higher arsenic content coppers from the Keweenaw Peninsula of Michigan, that you will have a hard time cleaning. It can be done, but with acids I don’t recommend using. Many of the folks in the Keweenaw will use a combination of acids and other chemicals to clean copper, and then as a final step, use Copper Brite (the acid dichromate solution Rock mentioned) to preserve the finish. But as Rock also mentioned; most serious collectors of native copper leave their specimens in a “natural” state with the patina intact.

If you just want to clean your copper, I would recommend placing them (2 or 3 at a time) in muriatic for no more than 5 minutes, then rinsing them off with water (a small pressure washer works well for this), drying them (I generally use a heat gun or hair dryer), then dipping them in Copper Brite. If you have crystalised specimens, I would leave them alone as most acids will ruin the sharp crystal edges by rounding them off...

I have quite a bit of copper and I am still trying to figure out a decent way to clean it. I tried a few small pieces in Tarn-X. They came out shiny, but turned iridescent in a matter of hours. A second soak resulted in quite a bit less iridescence. That was just a test, but it held promise for a resulting shiny piece with distinct edges. I have tried dilute HCl, but did not like the results at all. Any edges were rounded (as Paul mentioned) and the entire piece took on a hackly, shimmery appearance. With that test I ruined an interesting piece from the Phoenix Mine.

My problem is that I don't like the shiny "new" copper appearance. My favorite coppers are those that have the brownish patina to them, almost as if they had been kept in someone's pocket for a few decades. I have a decent amount of specimens like that, I am not willing to soak them in anything. Still, quite a few of them would exhibit more crystallization if I could find some way to clean them better. In some cases, the copper disappears into fractured calcite, or into matrix rock, but I can tell that there is more crystallization evident based on a few exposed surfaces.

I have tried mechanical cleaning, using small picks and a water gun. This does work in many cases, but it seems to take forever. On some pieces, it seems like it will take me 20-30 runs with this method to clean things out. The worst cases are those with copper and distinct calcite crystals, with matrix I want to remove. The copper is durable for the water gun, but I have to really be careful near the calcite.

When I get home, I will try to post a photo of what I am talking about. One piece that I have found particularly vexing falls into this category, and the piece almost seems to be wrapped with a granular matrix. I talked to some of the guys from the Caledonia mine at the last Detroit show about this piece, and they suggested continuing with the mechanical method. I think a lot of the pieces I have are just going to have to wait until I have a heck of a lot of time to work them over. I do have one piece at home that I really like, but I am not sure how it was cleaned. It does have a bit of a "new" look to it, but it is not overly shiny. More of a matte finish, and the crystallization is fantastic, though it is just a TN. Unfortunately, I can't find out how it was cleaned, as I found it at an estate sale quite a while back, and the original owner was not available.

Reports of copper dissolving in dilute HCl are puzzling to me since I first saw it in the book by Sinkankas. It is not supposed to be happening, is it?Dilute non-oxidizing acids, like HCl, at room temperature shall dissolve the surface coatings (oxide, sulfide) on copper but not the metal itself. Unless there is a sizable amount of a more electro-positive impurity (Ag, for example) that helps copper to corrode. But that is less likely to happen in the well-formed copper crystals.

Does someone have an explanation of the observed copper dissolution? What am I missing here?

You can try this method I was taught years ago by my high school science teacher. He showed me this cleaning technique on a copper penny which you might want to try first.

Place the tarnished penny in a suitable glass container. Cover the penny with white vinegar. Drop a small quantity of ordinary table salt into the vinegar and watch the penny magicallly turn bright and clean. Safely dispose of the solution and start again on one of your small copper specimens.

Rock Currier, Paul, Joe: Thank you very much for responding. I was going to take the photos today but the camera needed new batteries. I promise I will post the pictures tomorrow. Is there a limit on the number of pictures I can post? A size limit? My wife will have to upload/download? the pictures as I am not an intellectual when it comes to computers. Bill

When cleaning copper, collectors are usually concerned with degrees of brightness and the tone or shade of the patina they think is ideal. These are caused by slight changes in the surface chemistry of the copper, and I know of no real way to achieve a particular degree of brightness of oxidation other than trial and error. I also think that to some extent it all depends on the purity and amount of other elements mixed in to the copper from different localities. I think you will find that trial and error will be the only way to get the exact degree of tarnish you desire.

Okay, I didn't get a chance to post the photos yesterday, but I am getting around to it today. Unfortunately, I can't find the "particularly vexing" piece I referenced. I just realized it is in my camper, and I don't feel like going out there to search for it. While I am finishing my basement, especially my office and desk area, I have moved a bunch of items out there to store them including a few specimens I need to clean. Still, I have gathered a few others to post up, and I will try to get the other one tomorrow, or over the weekend.

This first one is the small TN I was referring to. It is about 3cm x 2cm, and has a bit of an iridescent sheen to it, and I believe it was chemically cleaned. Still, the crystals are pretty sharp, so however it was cleaned was a decent job. I am not very fond of the finish of the piece, but otherwise I really like it. The branching nature of the piece, along with the crystallization (and what I believe might be twinning) make this one of my favorite coppers.

By the way, that first piece is a Phoenix Mine specimen, for anyone interested.

This second one is a bit larger, but is an example of a piece I would like to clean a bit, but not change. It is about 25cm x 13cm, and I was told it came from "York Mine" or "New York Mine". Since I can't match that up exactly with an existing mine and there are several mines and prospects that reference New York and various parts of it (Brooklyn), I can only be sure it comes from Copper Country. This piece is an example of one that I would like to keep relatively intact. I really like the red and green patina to it, but would like to clean the gray material out of all the nooks and crannies. This is one that is taking some mechanical cleaning to get it looking better. Besides the green and red staining, there are some spots of light blue on it, though I am not sure what it is (the color is not right for chrysocolla or something along those lines) and there are some micro pockets of epidote on the bottom. This piece is an example of quite a few I have--where most of the dirt is gone, but there is some tough, hard material stuck in small pockets.

This last one is a partial copper crystal on matrix with copper throughout, about 5cm x 4cm, and is a bit more along the lines of the finish that I like. I have a few other pieces like this, but this is the only one that was handy at the moment. There is not much I would do here except perhaps a bit with the water gun on a lower setting.

I just noticed that Bill is posting as well. A lot of what he has is similar to material that I have, where quite a bit more cleaning is needed. On some of the pieces, you can tell there is copper crystallization, but it is hard to see with all the matrix making it look like a lump.

Scott,Did you collect the Phoenix specimen yourself? Reason I’m asking is are you sure that’s a Phoenix specimen? To me, it looks a lot like the Laker Pocket coppers that came out in 2008 which would have a brick red patina naturally. Your Indiana crystal is typical of what comes from there; partial/modified crystals with a tenorite coating.

The “York” piece and all of yours Bill are more in line of what comes out of the piles these days. One thing I failed to mention before is for the hard to reach spots, a can of Dow Scrubbing Bubbles does a pretty good job of loosening up the gunk without damaging anything, then water gun or pressure washer clean it the rest of the way. I’ve had pretty good success with this method. Another item that works quite well are the small spring loaded centre punches you can buy at Harbor Freight or Northern Tool. A few repeated pops usually will loosen the rock to a point you can chip it off fairly easily. Just be careful you don’t hit the copper as it will damage it quite quickly. As far as acids, muriatic is probably the safest and best “stronger” acid to use.

Bill, do you have any idea where your coppers are from (what mine)?? It would appear that a couple of those, especially the one in photo 010, are somewhat crystalised so you might want to be a little careful with those; the rest I would dip in muriatic for a couple minutes, remove, rinse, and repeat until you get the desired look. Some of those look like they will clean up nice with a little patience; others, I wouldn’t hold my breath on…..

Bill,I don't think you are going to find any chemical that is going to make all those specimens look really good. A little hydrochloric acid will remove any calcite that is on the specimens, but it will also probably remove any tarnish or oxidation that you may find attractive on the specimens. Then what you will most likely be left with are silicates like quartz, epidote and rock. The only way to effectively remove those would be through the use of Hydrofluoric acid and I am not sure I see any specimen in your group photos what would be worth the danger and expense of using that reagent. Even then about the best you could hope for is to strip the specimens down to the bare copper and have it really bright. Of course with time the color will tone down and oxidize. But of course there is no guarantee that they will oxidize to the color and or tone of your taste. You can experiment with fresh bright copper in a variety of ways like holding it over a flame and you may hit upon a method that will oxidize it just the way you want, but I won't be able to advise you there.

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